MIAMI, March 15 (Reuters) - The riverbank that hugs this city's downtown has in recent years sprouted flashy rows of towering high-rises and trendy restaurants. More boat slips are envisioned so water taxis can handle larger crowds.

But as business owners chart their course to prosperity, they have bumped up against a formidable adversary: Florida's much-loved "sea cow," the manatee.

When a county commissioner recently suggested relaxing guidelines intended to protect the blubbery creatures on the Miami River and other waterways, it set off the latest iteration of a perennial battle. Floridians are generally supportive of development, but they also adore their manatees, a gray, bulbous endangered aquatic species that weighs 800 to 1,200 pounds (363 to 544 kilograms).

"It's easy to draw battle lines, and it's really unfortunate," said Mark Bailey, executive director of the Miami River Marine Group, which includes marinas and shipyards that would like a chance to expand.

Native to Florida's rivers, bays and coastal waters, manatees have for decades been listed as an endangered species. They have no real defenses and cannot survive long in water colder than 68 degrees Fahrenheit (20 degrees Celsius).

Designated as the state marine mammal, the creature is a tourist draw in parts of the state where dozens of them cluster near warm springs and power plants. A National Park Service website described the manatee's appeal as that of "a plump grandmother with flippers like oven mitts, outstretched as if inviting a hug." Anyone who harms the so-called gentle giants risks arrest.

Last week two men pleaded guilty to manatee abuse after a Facebook video showed them luring manatees dockside and jumping on them in violation of the Florida Manatee Sanctuary Act.

The state law makes it a second-degree misdemeanor to "annoy, molest, harass or disturb" the manatee, an offense punishable by up to a year in prison and a $50,000 fine.

In 2012, a woman was arrested for "riding" a manatee near St Petersburg on Florida's west coast, according to an official report of the incident.

Many Florida counties have manatee protection plans, which need state and federal approval. Miami-Dade County's plan was adopted in the 1990s and is considered one of the state's best at protecting the creatures, said Katie Tripp, director of science and conservation for the Save the Manatee Club, co-founded by singer Jimmy Buffett and former Florida Governor and U.S. Senator, Bob Graham.

The state's manatee population has jumped from about 1,200 to nearly 5,000 in recent decades, and federal officials have mulled downgrading the creature's status from endangered to threatened. But last year, a red tide of toxic algea killed a record number of manatees, reminding everyone of their vulnerability.

Officials in Miami-Dade County count close to 200 sea cows. But manatees like to roam, and environmentalists say are more likely to swim through an area to feast on seagrass. Last year, 11 manatees died in Miami-Dade, two after being struck by watercraft, state figures show.

One of their preferred waterways is the Miami River, meaning that property owners there face restrictions on adding boat slips. Some business owners find those curbs outdated, they have told commissioners, because so many restaurants and condos are now crowded along the narrow waterway.

For several years, county officials have considered tweaking the manatee guidelines. Earlier this year, Miami-Dade County Commissioner Bruno Barreiro started the process to officially change some of them. "It's a very rigid plan," he said at a meeting this week.

His proposal would have kept the slow-speed zones that help prevent collisions between manatees and watercraft. But environmentalists expressed outrage at the extra boat traffic it could have added.

"They tipped the scales so far against the manatees," Tripp said.

County staffers were also skeptical that the plan would pass state muster, said Lee Hefty, the county's assistant director for environmental resources management, particularly since it loosened restrictions in spots where manatees and boats often cross paths.

Amid the outcry, Barreiro withdrew his proposal in hopes of reworking it to gain more support.

"Is it going to risk the life of even one extra manatee per year?" Commissioner Lynda Bell asked at a meeting on Thursday. If so, she said, the proposal wasn't worth it. ($1 = 0.6014 British Pounds) (Editing by David Adams and Gunna Dickson)

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